For many filtering applications it is usual to employ switched capacitor filtering arrangements. Besides requiring that each circuit be essentially custom designed for any particular application, such switched capacitor circuits have some disadvantages of their own. Switched capacitor circuits are good only as lowpass filters in wide band applications. A clock is required to control the corner frequency and the clock frequency often falls within the frequency band of interest. A further problem is that the clock often introduces undesired noise.
State variable circuits using operational amplifiers can also be used in some applications, but they require digital to analog converters. Since the number of digital to analog converters increases with filter order, such filters become very expensive as the filter order increases. It is therefore quite desirable to create a basic filter circuit which can be implemented into a single integrated circuit and which with very small changes can be used in a number of different applications.